This invention relates to apparatus for communicating by propagation of electromagnetic waves, and is broadly directed to apparatus for transmitting and/or receiving electromagnetic radiation over a broad range of radio frequencies. The invention is more particularly directed to a communications system utilizing a parametric magnetic dipole antenna which can be suitably mounted on platforms in a wide variety of environments, including on land, at sea, beneath the sea surface, a loft, in earth orbit, and in space.
At present, radio communication with divers or undersea vessels has been severly hampered by the difficulty of propagating radio waves in seawater.
Seawater, because of its relatively high conductivity, attenuates radio waves rather quickly. At radio frequencies useful in communication, radio waves are completely attenuated in less than about one wavelength. Generally, only so-called near-field propagation is possible--that is, a submerged vessel or diver cannot be more than about one radianlength (i.e.,.lambda./2.pi.) of the radio wave beneath the sea surface and still receive a useful amount of signal. Conseqently, for medium frequencies, a conventional submerged radio antenna needs to be within about one meter of the sea surface.
Recent attempts at subsea communication have involved use of the very low frequency (VLF) and extreme low frequency (ELF) radio spectra, i.e., those frequencies between 3 KHz and 30 KHz, and below 3 KHz, respectively. Because ELF and VLF radio waves have long wavelengths, the radio waves penetrate seawater to at least a useful distance. For example, a radio wave with a frequency of 10 KHz has a radianlength of about five kilometers, and, if that frequency is used, it is theoretically possible to communicate with a submerged vessel far below the surface.
Unfortunately, an electric dipole antenna for a frequency of 10 KHz, cut to one-half wavelength, would have an unwieldy length of about 15 kilometers (i.e., nine miles), thereby making the antenna impracticable for use on a any vessel. A long wire antenna could be cut to a somewhat shorter length, but would still need to be at least several thousand meters long to have an acceptably high transmission/reception power.
Consequently, a practical, small antenna useful at frequencies including the very low and extremely low frequencies has long been sought.